In reflection seismography it is common practice to employ at each ground station a plurality of geophones arranged in a grid pattern (array) and electrically connected in series-parallel. In this arrangement, vertically travelling compression waves (reflections) are received by all the geophones in the array simultaneously, while undesirable horizontal waves are received out of phase and are partially cancelled. For this method to work properly it is important that all the geophones in the array be matched in natural frequency, sensitivity and damping.
In conventional geophones, damping is accomplished in two ways. Most of the required damping is obtained by utilizing a conductive metal coilform, which acts as a low resistance shorted turn in the magnetic field. The remainder of the damping is provided by shunt loading of the sensing coil.
In the first way, using a conductive metal coilform, open circuit damping is provided by eddy currents induced in the metal coilform. Unfortunately, this is an effect that is difficult to control. Variations in the cross-sectional area of the coilform due to machining tolerances, variations in the electrical conductivity of the metal coilform alloy and ambient temperature changes, approximately a 4% resistance change for each 10.degree. C., all affect the resistance of the coilform and thereby the open circuit damping. Furthermore, coils wound on metal coilforms are subject to high voltage damage caused by breaking down the insulation between the winding coil and the coilform.
The present invention is directed to providing a geophone with a non-conductive coilform in which the coilform includes the properties of (1) lightweight whereby full damping can be accomplished solely by shunt loading of the sensing coil, (2) greater dielectric strength with greater protection against high voltage damage, (3) temperature stable, and (4) free of non-uniformities caused by conductive coilform damping, and (5) is a high strength and dimensionally stable plastic.